1. Field of the Invention
The present invention relates to a photoelectric conversion device, for example, to one-dimensional or two-dimensional photoelectric conversion devices for reading images in a video camera, X-ray imaging apparatus, infrared ray imaging apparatus, and the like and, more particularly, to a photoelectric conversion device using a skimming charge transfer scheme.
2. Related Background Art
Conventionally, CCDs are popularly used as imaging devices of video cameras, digital cameras, and the like, and such image reading device uses a "charge skimming transfer scheme" disclosed in, e.g., IEEE trans. Electron. vol. ED-29, p. 3, 1982 and Japanese Patent Publication No. 7-48826.
FIG. 13 shows an infrared ray imaging apparatus using the conventional charge skimming transfer scheme described in Japanese Patent Publication No. 7-48826.
FIGS. 12A and 12B show changes in charge amount before and after charge skimming transfer, and FIGS. 11A to 11C show the input circuit of a charge skimming transfer type infrared ray imaging element.
FIGS. 11A to 11C and FIG. 13 illustrate a photodiode 101, a silicon CCD 102, infrared rays 104, an output circuit 105, an input gate electrode 110, an accumulation electrode 111, a skimming electrode 112, a CCD electrode 113, an overflow drain 114, an overflow electrode 115, and a skimming voltage input terminal 119.
In FIGS. 12A and 12B, charges 116 are produced due to background radiation, charges 117 are ascribed to radiation from a signal source, and charges are skimmed at a skimming level 118.
The operation of the charge skimming transfer type infrared ray imaging element with the above arrangement will be described below with reference to the drawings.
(1) As shown in FIG. 11A, infrared rays 104 are converted into photocurrents by the photodiode 101, and the photocurrents are input to and accumulated in a portion beneath the accumulation electrode 111 via the input gate electrode 110.
(2) Upon completion of accumulation, as shown in FIG. 11B, a pulse signal is applied to the skimming electrode 112 to change the height of the potential well to transfer some accumulated charges to a portion underneath the CCD electrode 113. The amount of charges to be transferred is controlled by the pulse signal (to be referred to as a skimming voltage hereinafter) to be applied to the skimming electrode 112.
(3) Thereafter, as shown in FIG. 11C, charges remaining in the portion beneath the accumulation electrode 111 are ejected to the overflow drain 114 via the overflow electrode 115.
When charge skimming transfer is performed, as described above, since DC components 116 produced by the background radiation can be removed, as shown in FIGS. 12A and 12B, contrast can be emphasized, and the amount of charges to be transferred to the portion beneath the CCD electrode 113 can be reduced.
Charges transferred to the portion below the CCD electrode 113 are sequentially transferred by the CCD 102 in the same manner as in a case wherein the charge skimming transfer scheme is not adopted, and are output to an external circuit via the output circuit 105.
As described above, in the conventional photoelectric conversion device, when the number of photons of background radiation is extremely larger than the number of photons radiated by the signal source, the obtained signal has low contrast, and such problem cannot be solved by merely extending the accumulation time of the silicon CCD. In the above-mentioned prior art, this problem is solved by performing skimming charge transfer. However, since the skimming voltage is calculated and generated by an external circuit on the basis of the signal voltages of the respective pixels, other problems such as a long processing time, high cost of the entire system, and the like are posed.
On the other hand, an X-Y address type photoelectric conversion device, which uses a CMOS sensor or the like in place of the CCD and comprises a charge skimming transfer means, has not reached practical application level yet.